Absorption refrigeration and heat pump systems include a "generator" unit where refrigerant vapor is generated from a solution of refrigerant in absorbent. Absorption refrigeration and heat pump systems have generally heretofore employed pool boiling techniques for generating refrigerant vapor from the rich liquor (also called weak absorbent solution). Thus, in U.S. Pat. No. 3,270,523, a system is disclosed including a generator having a generator reservoir and a coil drawing rich liquor downward from a first position of the generator reservoir and helically upward around a burner and back to a second portion of the generator reservoir. The second portion of the generator reservoir is beside the first portion and separated from the first portion by a baffle. Refrigerant, which is present in a relatively high concentration in the rich liquor (weak absorbent solution), passes from the surface of the second portion of the generator reservoir into the analyzer portion of the generator and eventually to the condenser unit of the system. The solution which remains in the second portion of the generator reservoir, called weak liquor (or strong absorbent solution), is depleted in refrigerant. It should be noted that the weak liquor may theoretically be so depleted of refrigerant so as to be essentially pure absorbent, but the term "solution" is still used although not technically correct. The weak liquor passes from the second portion of the generator reservoir into an analyzer coil which passes in heat exchange relation with the first portion of the genrator reservoir and (in the analyzer section) with incoming rich liquor.
Such a pool boiling technique suffers several drawbacks. In absorption systems it is generally advantageous to boil the absorbent-refrigerant solution to the highest practical temperature in order to reduce the concentration of refrigerant in the absorbent solution as much as possible. The peak boiling point is often limited by the temperature of the boiler wall, however, because corrosion or decomposition reactions at the boiler wall may be the factor limiting the peak-operating temperature, or may require the use of unacceptably expensive materials of construction to prevent such reactions.
To produce boiling, a significant minimum temperature differential (.DELTA.t) is required between the heat transfer surface and the body of the liquid. Known properties of boiling liquids, illustrated for example on page 13-2 of Handbook of Heat Transfer by W. M. Rohsenow and J. P. Hartnett (New York 1973), indicate that for pool boiling a minimum boiling .DELTA.t is required even at low heat transfer rates. The minimum .DELTA.t is a function of the fluids being boiled. If boiling occurs at a normal smooth surface, the .DELTA.t's may vary from 10.degree. to over 100.degree. F. depending on the fluid. When the wall temperature determines the potential for corrosion or decomposition, as is the case for many absorption pairs (refrigerant and absorbent), the maximum usable liquid temperature may therefore have to be much lower than the limiting wall temperature. In experiments conducted with dichloromonofluoromethane as refrigerant and ethyl tetrahydrofurfuryl ether as absorbent, .DELTA.t's of 70.degree.-90.degree. F. were found to be necessary at the desired heat fluxes in a normal smooth vertical tube of 6 inch diameter.
One method of reducing the boiling temperature differential is to use the "boiling chips" common in organic laboratory and distillation processes. The Linde HYFLUX.RTM. boiling surface is a variation on the boiling chip process. It utilizes a porous metal film sintered on the boiler surface. The HYFLUX surface is highly effective in reducing the boiling .DELTA.t when boiling pure liquids. However, when a solution is being boiled, as in an absorption system, the reduction of .DELTA.t is much less. The reduced effectiveness in boiling solutions is presumed to be due, at least in part, to the high boiling liquid remaining in the pores as the low boiling component is evaporated off.
Another means of producing high boiling heat transfer rates at low t is to utilize forced convection of the boiling liquid. The gains available by using forced convection are indicated qualitatively on the upper left of the chart on page 13-2 of the Rohsenow and Harnett book. It is indicated that higher transfer rates are possible at lower temperature differentials than occur with pool boiling at high .DELTA.t's. Normally, if forced convection were used, it would require the application of some means of stirring the liquid, with mechanical stirring being undesired because of the power requirements and the need for means for transferring the power into the hermetical system without the use of seals which could leak. To eliminate the potential drawbacks of mechanical stirring, the present invention utilizes the high liquid velocities produced in vapor lift action within vapor lift or "percolator" tubes to produce forced convection boiling with no moving part.
Up to five advantages or objectives are achieved in the practice of the present invention, compared to pool boiling systems: (1) the boiling heat transfer rates are increased in all embodiments of the present invention thus reducing the temperature differential below that required for pool boiling; (2) the boiler wall temperature is decreased in many embodiments at locations where the method described in #1 is less effective; (3) the boiler is caused in most embodiments to have a fractionating effect within itself; (4) the residence time of the boiling liquid at its peak boiling temperature is minimized to a varying extent in the several embodiments; and (5) in some embodiments the fin surfaces are attached to the boiler in a manner producing a safeguard against overheating of the boiler.
Pump tubes have on occasion been suggested for use in the generator portion of an absorption system. For example in FIG. 9 of U.S. Pat. No. 3,516,264 (June 23, 1970 to Stierlin) a "pumping tube" is provided with heating means at its lower end only. Liquid is driven upward by refrigerant bubbles rising, but no further heat is applied to the fluid in the upwardly extending conduit. Pump tubes of sorts are disclosed in U.S. Pat. Nos. 1,729,355 (Sept. 24, 1929 to Munters) 1,791,441 (Feb. 3, 1931 to Bertsch), 1,886,243 (Nov. 1, 1932 to Gordon), 2,480,497 (Aug. 30, 1949 to Meyer), 2,557,573 (June 19, 1951 to Sherwood), 2,617,632 (Nov. 11, 1952 to Simpson), 2,625,801 (Jan. 20, 1953 to Whitlow) and 2,625,802 (Jan. 20, 1953 to Whitlow).
Finned generator devices are also known, as for example in U.S. Pat. Nos. 2,306,704 (Dec. 29, 1942 to Kogel), 3,254,507 (June 7, 1966 to Whitlow) 3,367,310 (Feb. 6, 1968 to Whitlow et al.), 3,407,625 (Oct. 29, 1968 to McDonald). A fire tube furnace with circumferentially spaced holes is disclosed in U.S. Pat. Nos. 3,895,607 (July 22, 1975 to Johnson) and 2,030,265 (Feb. 11, 1936 to Nygaard).